1. Field of the Invention
The present invention relates to the field of curable compositions such as adhesives that include a conductive component and more particularly to acrylate, methacrylate and cyanoacrylate adhesives including such a component.
2. Brief Description of Related Technology
Many curable formulations, particularly cyanoacrylate—based ones, are cure sensitive to components one might introduce to such formulations. For example it is often desired to include components that will modify the physical properties of the formulation, for example tailoring the physical properties to a desired end-use application. It is often desired to include other components in adhesive formulations for various other reasons. It is sometimes difficult to incorporate these materials in the material without compromising at least one property of the composition, such as stability, for instance shelf life stability. On the other hand rapidly curing compositions are desirable for at least certain end-use applications.
Many techniques have been used to introduce additional materials into curable compositions without affecting the formulation stability. One of these ways is to encapsulate the component in some form, for example by the formation of microcapsules which hold at least one component internally and which operate as a physical barrier to separate components which might otherwise react to cause curing. Often a polymerizable monomer and/or a curative therefor are separated by microencapsulation.
One component that is often introduced into a composition is a conductive component. Many types of conductive components are known, many of which being in the form of particles. The particles may be regularly shaped or in the form of flakes. Often the conductive particle(s) is (are) used to create one or more conductive paths. Where a curable composition is used to bond two components together it is often at times desirable that the composition does not form (when cured) an insulator between components. For example when bonding substrates, such as where an electronics component is to be bonded to a circuit board [such as a printed circuit board (“PCB”)] it is usually desirable that the composition when cured conducts electricity between the substrates.
It is desirable also in many cases for the formulation to form one or more anisotropic conductive pathways. In particular it is desirable that the composition does not short circuit the component assembly in any way (for example shorting between the pins of an electronic component). Many formulations have been devised where anisotropic effects have been achieved, of which some include non-random ordering of the particles. Many of these are formed in (conductive) monolayers for example using magnetic ordering.
U.S. Pat. No. 6,149,857 (McArdle) describes an anisotropically-conductive film or a substrate having a surface coated with an anisotropically-conductive coating. The film or coating is formed by solidifying a composition comprising (i) a solidifiable ferrofluid composition and (ii) a plurality of electrically-conductive particles, dispersed in the ferrofluid. The ferrofluid comprises a colloidal suspension of ferromagnetic particles in a non-magnetic carrier. The electrically-conductive particles are arrayed in a non-random pattern by application of a substantially uniform magnetic field to the composition in a liquid state and are locked in position by solidification of the composition. The composition is solidified in an A-stage, usually involving a primary cure. In end-use application of the film or coating, the composition usually undergoes a B-stage or secondary cure. The film or coated substrate is an article of manufacture for bonding conductors in the electronics industry. Similar technologies are described in U.S. Pat. Nos. 6,110,399, 5,851,644, and 5,769,996 all to McArdle.
In an unrelated field relating to topical transdermal treatments, European Patent Publication No. EP 1 043 018 (L'Oreal) discloses a patch which comprises a polymeric matrix and with an adhesive surface. The patch also includes dispersed magnetic particles. A method of preparation of the particles is also disclosed.
In another unrelated field of dental compositions, Great Britain Patent Publication No. GB 1,122,439 (Mitsuharu Takeuchi) describes the incorporation of dentally acceptable solid material in dental compositions. In the Examples monomeric ethyl cyanoacrylate is placed in pits and fissures in the teeth. Where the cyanoacrylate contacts a surface it is described as polymerising instantly while other of the material remains pasty in consistency. To the part of the composition which remains pasty a powdery mixture of methyl methacrylate polymer and metallic particles (such as gold) are added. The composition hardens within a minute after which time it is shaped. Coating of the particles is also mentioned. Coating is achieved by placing particles in an amount of liquid methyl methacrylate, heating, drying and pulverising.
U.S. Pat. No. 4,452,861 (Okamoto) describes solid particles encapsulated with cyanoacrylate polymer. The particles are encapsulated to prevent degradation due to reactive or corrosive surroundings. The cyanoacrylate polymer is used to coat phosphor particles and the like which are employed as coatings in cathode ray tubes and the like. Cerium activated calcium sulphide phosphor powder is the exemplified material which is coated.
Notwithstanding the state-of-the-technology, it would be desirable to provide a curable composition that is suitable for use in bonding electronic components which has a fast cure profile and desirable end-use physical properties. In particular it is desirable to provide a curable composition that can form conductive bonds between two substrates that are bonded together by the composition.